A plug assembly is known that has a socket having a housing, an outer sleeve on the housing of a predetermined inner diameter, centered on an axis, and having an axially extending and radially inwardly open groove, and a plurality of contacts mounted in the outer sleeve. A switch on the housing is movable between an on position and an off position and a latching element in the housing in axial alignment with the groove is displaceable axially relative to the outer sleeve between inner and outer positions. A spring or other biasing means is braced between the element and the housing for urging the element into the outer position and a coupling between the latching element and the switch prevents the switch from moving from the off position to the on position except when the element is in the inner position. A plug has a body with an outer diameter generally equal to the inner diameter of the outer sleeve. Respective contacts in the plug body are engageable with the contacts of the outer sleeve so that when the plug body is fitted inside the outer sleeve the plug contacts engage the socket contacts. A radially outwardly projecting bump on the second-plug body engages the latch element to hold same in the inner position in the inserted position. Such a system with electrical decoupling is described in German patent document 2,948,843.
Thus only when the plug is inserted fully into the socket can the switch be moved to the on position. In a variant on the system a latch is provided for locking the plug in the socket when the switch is in the on position so that the socket terminals cannot therefore be left hot and unexposed, and the danger of arcing on disconnect is completely eliminated. Thus this arrangement is quite safe. In addition the position of the alignment bump and the array of the contacts can ensure that plugs and sockets are matched perfectly, making misconnections virtually impossible.
The main problem is that a piece of equipment having the state-of-the-art matched sockets is frequently used with older add-ons whose plugs do not match. Thus it is necessary to replace the cables at least of the auxiliary equipment.